1. Field of the Invention
This invention relates to an image forming apparatus such as a photocopier and a printer and, more particularly, to an apparatus at which a mark for detecting a position is formed on a moving body.
2. Description of Related Art
In image forming apparatuses, various methods such as an electrophotographic method, a thermal transfer method, an inkjet method are used conventionally. Apparatuses using the electrophotographic method, inter alia, are excellent in comparison with other methods in terms of high speed, high image quality, and noiselessness, and are widely used as image forming apparatuses such as photocopies and page printers. With the electrophotographic method, images made of toner particles are formed on a transfer material by utilizing electrostatic force, and the toner image is then melt and fixed onto the transfer material according to beat and pressure applied by a fixing device, and enters into a stable state. Recent image forming apparatuses of the electrophotographic type have features of advanced functions such as color application and high speed.
FIG. 6 is a schematic diagram showing an image forming apparatus using an electrophotographic method as a prior art. A multicolor image forming apparatus using an intermediate transfer method is shown in FIG. 6. In the drawing, a photosensitive drum 101 as an image bearing member is uniformly charged with a charging roller 102, and scanning a scanning beam 104 with an exposure optical system 103 forms a latent image. A toner image is formed to the latent image upon development with a developing device 105, having a developing roller 105a facing the photosensitive drum 101, attached to a rotary member 122, and is primarily transferred onto an intermediate transfer belt 109 as an intermediate transfer body from a primary transfer roller 110, which is charged by a power source 119, at a primary transfer nipping portion N1. The primary transfer remaining toner remaining on the surface of the photosensitive drum 101 after the primary transfer is removed by a photosensitive drum cleaner 117 having an elastic blade, and is contained in a waste toner container, not shown.
During the full color image formation (or full color mode), the above steps are sequentially performed with respect to developing devices 105 to 108 of the respective colors in synchrony with the rotation of the intermediate transfer belt 109, thereby forming a color image overlapped with four colors on the intermediate transfer belt 109. It is to be noted that during the monochrome image formation (monochrome mode) the above steps are done only for the black developing device to reduce the throughput.
The toner image thus formed on the intermediate transfer body is transferred secondary to the transfer material P at once by a secondary transfer roller 111 at a secondary transfer nipping portion N2 as a position for secondary transfer. The toner remaining on the intermediate transfer belt after the secondary transfer, is removed by a intermediate transfer body cleaning mechanism (whose detail will be described below). Then, the toner image is melt and fixed to the transfer material P in application of heat and pressure at a fixing device 121, and the transfer material is delivered to the exterior of the apparatus to end the image formation.
It is to be noted that the intermediate transfer belt is tensioned among a tension roller 116, a drive roller 115, and a secondary transfer opposing roller 112. A position detection mark (hereinafter referred to as “detection mark”) 145 for detecting the position of the belt is adhered to an end of the belt, and a detecting sensor 146 is arranged at a position detectable of the detection mark near the intermediate transfer belt. The operation timings of the respective portions in the apparatus are determined based on the timing that the detecting sensor 146 detects the detection mark 145 to form the images at a prescribed position on the intermediate transfer belt.
So called tandem type color image forming apparatuses, other than the above apparatuses, have been known in which an image forming station containing such as a photosensitive drum and a developing device and being capable of developing a monochrome toner image, is provided independently for each color and in which such image forming stations are disposed on the periphery of the intermediate transfer belt. In the tandem method, the intermediate transfer body surface sequentially passes through the respective image forming stations to primarily transfer the respective toner images in an overlapping manner on the intermediate transfer body, thereby forming the full color image on the intermediate transfer belt. The secondary transfer and fixing operation, which are done subsequently, are performed in the same way as the prior art described above.
Next, the cleaning mechanism for intermediate transfer body is described. Secondary transfer remaining toners remaining as not transferred to the transfer material P are attached to the intermediate transfer belt 109 at a state that the secondary transfer step is completed. Conventionally, a remaining toner charging roller 131 is disposed at the intermediate transfer belt 109, and the charging roller charges the toner to be at a plus polarity as the polarity opposite to the toner's original charging polarity (charging polarity after toner development). The charged secondary transfer remaining toner is then transferred to a photosensitive drum 101 at a primary transfer nipping portion N1 as the primary transfer position and is removed from the intermediate transfer belt 109. It is to be noted that the secondary transfer remaining toner moved onto the photosensitive drum 101 is removed by the photosensitive drum cleaner 117 in substantially the same manner as the primary transfer remaining toner.
With this cleaning mechanism, if the charged amounts possessed by the respective particles of the charged secondary transfer remaining toner are not substantially even, adequate cleaning capability may not be obtained. The toner particles having a low charge amount receive weak electrostatic force from the electric field formed at a primary transfer nipping portion N1, and are transferred not easily to the photosensitive drum 101. The toner particles having a high charge amount receive large mirror reflection force from the intermediate transfer belt 109, and are therefore transferred not easily to the photosensitive drum 101 as a matter of course.
In a prior art, a bias in which an alternative current voltage is convoluted to a direct current voltage is used as the bias applying to the remaining toner charging roller 131 from a power source 137. This bias is excellent in ability for uniformly charging toners in comparison with the direct current bias, so that the respective charging amounts possessed by the respective toner particles can be made substantially uniform, and so that the apparatus can obtain adequate cleaning capability.
When the remaining toner charging roller 131 being applied with the above alternative voltage charges the secondary transfer remaining toner, however, toner scattering may occur around a contacting portion N3 between the remaining toner charging roller 131 and the intermediate transfer belt 109, thereby possibly messing up the interior of the apparatus. The secondary transfer remaining toner repeats jumping between the remaining toner charging roller 131 and the intermediate transfer belt 109 at the contacting portion N3 upon reception of the influence of the alternative current voltage. During this jumping process, although the respective toner particles of the secondary transfer remaining toner are charged at the positive polarity uniformly, the particles particularly with a lower charged amount among the secondary transfer remaining toner may not reach the remaining toner charging roller 131 and cannot come back to the intermediate transfer belt 109. According to the gravity and air flow caused by the rotation of the intermediate transfer belt 109, particles may float or drop down, thereby causing toner scattering.
To solve this problem, a method has been devised in which the secondary transfer remaining toner is charged at a plus polarity prior to charging operation by means of the remaining toner charging roller 131 in use of an auxiliary charging member 140 to which direct current voltage from a power source 141 not causing any scattering only applies. This renders the toner having a low charging amount which otherwise maybe scattered near the contact N3 of the remaining toner charging roller, so applied with an adequate charging amount in advance as not to be scattered, so that toner scattering hardly occurs. It is to be noted that opposing electrodes 132, 135 coupled to the ground level are formed on back surfaces of the remaining toner charging roller 131 and the auxiliary charging member 140 via the intermediate transfer belt 10 to raise charging efficiency.
Where the first charging means (auxiliary charging roller) contacting to the surface of the intermediate transfer belt, as an auxiliary charging member, is used, however, the following problems occur as illustrated in FIG. 7. FIG. 7 is a cross section around the intermediate transfer belt 109 at the auxiliary charging member 140.
When the detection mark 145 on the intermediate transfer belt 109 passes though the contact nipping portion between the auxiliary charging member 140 and the opposing electrode 135, if the auxiliary charging member 140 rides on the detection mark 145 and if the opposing electrode 135 supports the portion of the intermediate transfer belt at which the detection mark 145 is located, a portion of the roller may come afloat from the intermediate transfer belt, thereby rendering toner scattering and cleaning defects occur due to uncharged transfer remaining toner at that portion.
Such floating of the auxiliary charging member 140 remarkably happens especially where the auxiliary roller has a high hardness, more specifically, where the roller has a hardness of 45 degrees or more on JIS-A hardness scale (i.e., hardness of the roller single measured directly with a JIS A hardness measuring device), and further likely occurs where the pressure (contact pressure) pushing the auxiliary roller or the intermediate transfer belt is weak or where the detection mark has a thickness of 30 microns or more.
In a meantime, the parts in the apparatus are sought to be compact and made with lower costs according to high demands on compact and inexpensive apparatuses these days. It is advantageous to make the hardness higher to satisfy the durability and strength with a more inexpensive roller. Moreover, where the roller is made with a smaller diameter, the elastic layer is reduced to inevitably render the roller harder, and since the strength is lowered, it is required to reduce the contact pressure to the intermediate transfer belt (or the opposing electrode) so as not first charging means (the auxiliary charging roller) as far as not subjecting to occurrence of floating and can be used, so that improvement on the above point is desired.